Load control and scheduling are two important functionalities in a Wideband Code Division Multiple Access (WCDMA) system. Load control and scheduling are required to keep the resource usage at a desired level, while maximizing usage of the available resources. Load estimation is one basic module in these two functionalities which estimates the (radio) resource usage.
For the uplink, the common radio resource shared by the mobile stations is the total amount of tolerable interference, which can be defined as the average interference over all the antennas. A relative measure of total interference is rise over thermal, i.e. total interference relative to thermal noise.
Further, the load factor represents the portion of uplink interference that a certain channel of a particular mobile station generates, which can be defined as the interference on the channel caused by the mobile station divided by the total interference. The total load factor of different channels equals to the sum of load factors due to different channels. Accordingly, uplink load control determines for each cell the maximum available load room that can be used by the scheduling function based on the uplink interference situation in that cell. Uplink scheduling determines for each cell the maximum data rate that can be supported given the maximum available load room, which is also called load headroom to rate mapping. This is typically implemented by first determining the supportable power offset between the channel to be scheduled (or requiring rate increase) and the Dedicated Physical Control Channel (DPCCH) channel, which is a fixed rate channel, see third generation partnership project (3GPP) Technical specification TS 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD) (release 7)”, V7.2.0, and as shown in formula (1):pwroffgrant—estk=f(loadavik,loadDPCCH—estk,loadsched—estk)  (1)
Where:                pwroffgrant—estk is the (estimated) power offset that can be granted for the channel to be scheduled (or requiring rate increase) for user k        loadavik is the load room available for user k, which equals to the maximum available load room minus the total (estimated) load from the users that are transmitting.        loadDPCCH—estk is the (estimated) DPCCH load from user k. Note that the DPCCH load will vary when the data rate of the other channel (belong to the same user) is changed.        loadsched—estk is the (estimated) load from the channel(s) that are already scheduled for user k.        
The supportable data rate is then determined based on the granted power offset. The DPCCH load will decrease for a user that is granted more data rate. These two control functionalities are performed iteratively to cope with e.g. varying traffic or channel conditions.
Uplink load estimation estimates the load that has been or will be generated in each cell for different channels. The accuracy of the uplink load estimation is crucial to make sure that the uplink load control and the uplink scheduling work as desired. Any error in the uplink load estimation may lead to:
Load overestimation, resulting in that too conservative data rate is granted, consequently insufficient resource usage and also throughput loss
Load underestimation, resulting in that too aggressive data rate is granted, consequently excessive resource usage. This may also lead to uplink instability, i.e. mobile station power and rise over thermal oscillation, which is especially harmful to users with high QoS requirement (fixed rate and/or short delay, etc.) services.
The uplink load can be estimated based on either the Carrier to Interference Ratio (CIR) (or equivalently, the date rate) or the wide band received power (according to the original definition of load factor), see H. Holma and A. Toskala, “WCDMA for UMTS, Radio Access for Third Generation Mobile Communications”, Chichester, UK: Wiley, 2004.
Suppose user k has N uplink channels, formula (2) below is currently used for the CIR based load estimation:
                              Load          i_est          k                =                                            CIR                              1                ⁢                _meas                            k                        ·                          pwroff              i_est              k                                                          loadpar              1                        +                                          loadpar                2                            ·                              CIR                                  1                  ⁢                  _meas                                k                            ·                              (                                  1                  +                                                            ∑                                              i                        =                        2                                            N                                        ⁢                                          pwroff                      i_est                      k                                                                      )                                                                        (        2        )            
Where:                CIR1—meask is the (measured) CIR of the 1st channel from user k        pwroffi—estk is the estimated) power offset between the ith channel and the 1st channel of user k        Loadpar1 is the first load estimation parameter.        Loadpar2 is the second load estimation parameter.        
These two load parameters are currently fixed and set on the Radio Network Controller (RNC) level. However, in reality they are varied with radio environment, receiver scheme, and also data rate and interference levels (for some kind of receivers for example Generalized RAKE receivers (GRAKE)) etc.
For other types of receivers, the actual load expressions are more complicated. This is true for e.g. Successive Interference Cancellation (SIC) receivers and GRAKE+ receivers. In this case, the load depends on several factors, all of which are difficult to take into account in load formula like (2). One solution is to introduce a simpler expression that is accurate only in a limited region close to the current operating point. Such an expression may serve as a first order estimation of the load contribution, e.g.,Loadi—estk=CIR1—measkpwroffi—estk·loadpar  (3)
In this case, it is acknowledged that the load parameter loadpar changes when e.g. the User Equipment (UE)/Mobile station rate changes. Using (2) for predicting the load may be more accurate than using (3) for some types of receivers. On the other hand, (3) is applicable to a wider range of receivers.
For power based load estimation, the load factor due to the ist channel of user k is estimated as:
                              Load          i_est          k                =                              pwr            i_meas            k                                Itot            meas                                              (        4        )            
Where:                pwri—meask is the (measured) wide band received power generated by the ist channel of user k        Itotmeas is the (measured) total interference        
As set out above for CIR based load estimation, the load parameters are fixed, but in reality they may vary. This variation may cause load estimation error and large rise over thermal oscillation. To ensure system stability fairly large load margin is needed which may negatively impact the system throughput. Furthermore, it is difficult to configure the load parameters and load margins that are suitable for all scenarios while a too conservative setting will decrease the system performance.
Moreover, inaccurate load estimation (even in average sense) make it difficult to implement resource utilization based scheduling schemes (e.g. resource fair scheduling or minimal resource utilization scheduling). This limits the performance of the scheduling functionality.
One solution to this is to dynamically adapt the load parameters. However, exact expressions of the load parameters depend on the receiver schemes. Even the CIR based load estimation formula itself may have different forms other than formula (2) when receivers other than RAKE(2) and GRAKE(2) are used, and it is hard to derive the load estimation formula as well as the expressions for the load parameters for some receivers, e.g. GRAKE type 3, successive interference cancellation (SIC), etc. Consequently it is hard to adopt the load parameters adaptation directly based on their theoretical expressions.
This may even be impossible in some cases, such as Uplink (UL) Coordinated Multi Point transmission/reception (CoMP). In UL CoMP, the number of antenna signals that are used during the detection can change based on how many links are strong. In effect, this leads to a change in the diversity factor for the connection. This can be observed as the actual receiver type is updated, going from e.g. GRAKE2 to GRAKE4.
Another solution is to control the received power instead of CIR, either the DPCCH received power or the total received power, in inner loop power control. In this way the rise over thermal oscillation is inherently avoided even with fixed load parameters. However, it is crucial to ensure the quality of DPCCH which provides the reference for channel estimation and demodulation, etc. Therefore it is not possible to simply discard the CIR based power control for DPCCH, and in order to control the total received power another control loop needs to be introduced, which is more complicated.
For power based load estimation, the corresponding load estimation formula (formula (4)) is independent of the receiver schemes and the variation in radio environment, receiver schemes, etc. is automatically reflected in the wide band received power. This is because the uplink (inner loop) power control always tries to adjust the User Equipment power so that the perceived CIR at the base station is stable, i.e. close to the CIR target. The level of received signal power will however, depend on the receiver scheme, data rate and radio environment.
However, the variation in power also leads to that the power based load estimation is more sensitive to delays than the CIR based load estimation, especially for high data rate users with high transmit power.
Moreover, the power based load estimation does not consider that DPCCH load will decrease when more date rate is granted, but assumes fixed DPCCH load during the load headroom to rate mapping. This leads to that data rate is under-granted and many iterations are typically required to get the desired data rate.
Hence, there exists a need for a method and a system that provides an improved determination of the uplink load.